A common method of completing a subsea well involves installing a high pressure wellhead housing at the sea floor. Multiple strings of casing will be supported at the wellhead housing and extend into the well. Then a tubing hanger with a string of tubing and other downhole equipment such as a packers, will be run into the casing and landed in the wellhead housing. The steps of drilling through the high pressure wellhead housing, running the casing and running the tubing are performed through a drilling riser and blowout preventer.
The tubing hanger running tool will normally be attached to a dual completion riser which has one conduit in communication with a production bore in the tubing hanger and another conduit in communication with an annulus bore in the tubing hanger. The production and annulus bores are parallel to and offset from each other. Tubing annulus communication is needed for certain operations during completion. After the tubing string has been landed and the tubing hanger set, the operator runs plugs on wireline through the completion riser conduits, one plug sealing the annulus bore and the other plug sealing the production bore.
The operator then removes the dual string completion riser and the drilling riser along with the blowout preventer. The operator then runs a Christmas tree, landing the tree on the wellhead housing and completing the well. The wireline plug(s) in the production bore and annulus are removed for production.
It is expensive for a drilling rig to have a dual string completion riser in addition to a drilling riser. It would be preferable to be able to run the dual bore tubing hanger through the drilling riser on a single monobore conduit such as a string of drill pipe. Running on a drill string, however, does not readily allow a wireline plug to be installed in the annulus bore in the tubing hanger, because the drill string bore is aligned with the production bore. Tubing hangers with various valves for the annulus have been proposed, but have not been used extensively because of reliability concerns. Check valves have been used in the tubing hanger annulus bore in the past, but are not in general use because of reliability concerns and because of the inability of being able to test from above prior to removing the blowout preventer.